This invention relates generally to ion and plasma sources, and more particularly it pertains to gridless or Hall-current ion sources.
Industrial ion sources are used for etching, deposition and property modification, as described by Kaufman, et al., in the Characteristics, Capabilities, and Applications of Broad-Beam Sources, Commonwealth Scientific Corporation, Alexandria, Va. (1987).
Both gridded and gridless ion sources are used in these industrial applications. The ions generated in gridded ion sources are accelerated electrostatically by the electric field between the grids. Only ions are present in the region between the grids and the magnitude of the ion current accelerated is limited by space-charge effects in this region. Gridded ion sources are described in an article by Kaufman, et al., in the AIAA Journal, Vol. 20 (1982), beginning on page 745. The particular sources described in this article use a direct-current discharge to generate ions. It is also possible to use electrostatic ion acceleration with a radio-frequency discharge.
In gridless ion sources the ions are accelerated by the electric field generated by an electron current interacting with a substantial magnetic field in the discharge region. The overall size and weight of a gridless source is primarily determined by the magnetic circuit to generate this magnetic field. A substantial fraction of the overall cost of a gridless ion source is also associated with the magnetic circuit. In contrast, when a magnetic field is used in a gridded ion source, it is only to contain the 50 eV, or less ionizing electrons. The magnetic circuit in a gridded ion source thus plays a secondary role to the ion optics in determining ion-source size and cost.
Because the ion acceleration takes place in a quasineutral plasma, there is no space-charge limitation on the ion current that can be accelerated in a gridless ion source. The lack of a space-charge limitation is most important at low ion energies, where a gridded ion source is severely limited in ion-current capacity.
The closed-drift ion source is one type of gridless ion source and is described by Zhurin, et al., in an article in Plasma Sources Science and Technology, Vol. 8, beginning on page R1, while the end-Hall ion source is another type of gridless ion source and is described in U.S. Pat. No. 4,862,032xe2x80x94Kaufman, et al. These publications are incorporated herein by reference.
A Hall current of electrons is generated normal to both the applied magnetic field and the electric field generated therein, so that these ion sources have also been called Hall-current sources. Because the neutralized ion beams generated by these ion sources are also quasineutral plasmas, i.e., the electron density is approximately equal to the ion density, they have also been called plasma sources.
Gridless ion sources used in industrial applications need routine maintenance. This maintenance can result from the limited lifetimes of certain parts, such as cathodes. The need for maintenance can also result from the contamination of ion-source parts due to sputter deposition within the ion source, or from the contamination with materials present in the particular application in which the ion source is used. The contamination can be in the form of conducting layers on insulators, insulating layers on conducting parts, or deposited films that can peel off to cause electrical shorts or flake off in smaller particles to generate unwanted particulates.
Performing the routine maintenance typically involves replacing cathodes and some other parts with limited lifetimes, cleaning the remaining metal parts, and replacing insulators. The ion sources must be substantially disassembled to carry out this maintenance.
The expense of performing maintenance on gridless ion sources is not limited to the direct time and materials involved. The downtime for the vacuum chamber and associated hardware often constitutes a major expense. This latter expense can be reduced by purchasing two ion sources, so that maintenance can be performed on one ion source while the other is being used. However, the purchase of an additional ion source is an additional expense that must be balanced against the reduction in downtime expense.
In light of the foregoing, it is a general object of the invention to provide a gridless ion source with a detachable anode module that facilitates rapid and economical maintenance.
A specific object of the invention is to provide a gridless ion source with a detachable anode module in which the cost of that module is substantially less than the expense of the entire ion source.
Another specific object of the invention is to provide a gridless ion source with a detachable anode module in which the size and weight of that module is substantially less than the size and weight of the entire ion source.
A further specific object of the invention is to provide a gridless ion source with a detachable anode module in which the contamination of ion-source parts due to sputter deposition within the ion source, and the associated maintenance, is essentially confined to that module.
Yet another specific object of the invention is to provide a gridless ion source with a detachable anode module in which the deposition on ion-source parts due to contamination sources external to the ion source are largely confined to that module.
Still another specific object of the invention is to provide a gridless ion source with a detachable anode module in which the loss of working gas is minimized by a gas enclosure surrounding the anode in that module.
In accordance with one embodiment of the present invention, the ion-beam apparatus takes the form of an end-Hall ion source in which the detachable anode module incorporates the outer pole piece and includes an enclosure around the anode that both minimizes the loss of working gas and confines sputter contamination to the interior of this enclosure. This detachable anode module is substantially smaller than the entire end-Hall ion source, weighs substantially less, and can be duplicated for significantly less cost than the duplication of the entire ion source. In general, the components of the magnetic circuit determine the overall size, weight, and much of the cost of a gridless ion source. The reduced size, weight, and cost of the detachable anode module compared to the entire ion source is due to most of the magnetic circuit being excluded from the detachable module.